Antiquated Key

The Anitquated Key is found in the Lost Bastille.
It is found in the same chest as the Covetous Silver Serpent Ring. From the Tower Apart bonfire, go to the building
on the left. Move to the lower floor and go outside and look for a metal chest. This chest contains the key.

This key can be used to open up a shortcut bypassing the
Ruin Sentinels boss fight. From the
Exile Holding Cells bonfire, go through the Bastille
ramparts, and head down to the main grounds. Look for a
boarded up doorway and smash through it. Go straight
through the main courtyard and look for a ladder in the leading up to the windows of the building in front. Go
through the windows and turn right. This will lead to
a room with a Pharros contraption. Activate the contraption,
and in the left corner of the back of the room will
reveal a Pharros illusory wall. Smash through this and this
will open up a secret room with a metal chest and a cage lift.
Take this cage lift upward, and this will lead to the
building with the Straid's Cell bonfire as well as
the walkway to get to the Sinner's Rise.